This invention relates to the preparation and use of asymmetric hydrophobic membranes. While asymmetric hydrophilic membranes that preferentially remove water are known, the preparation of the elastomeric asymmetric hydrophobic membranes which act preferentially in transfer of organic compounds is new. The invention represents an advance in technology related to separation of organics from other organics and from water, from contaminated solutions and from air or other gaseous or vapor mixtures. The methods of the instant invention are more cost-effective than those previously available.
Until this time, the state of the art of the technology to produce pervaporation or vapor permeation membranes has been based on the formation of composite membranes. A composite membrane is composed of a porous support layer used to provide the physical support of the active membrane film, the thin second layer that is added to provide for the separation of the species of interest. While hydrophobic polymers such as those used herein have been known, the prior art comprises their use in composite membranes composed of a porous support such as a porous polymer, ceramic or metal support in either a flat (plate or film) or tubular shape which is overlain by a very thin, dense polymer film require several steps for their manufacture: (1) preparation of the support layer, (2) preparation of the membrane layer and (3) overlaying of the support with the membrane. There is need to reduce cost of both construction and use of hydrophobic membranes.
The first asymmetric (hydrophilic) membranes were developed in the late 1950's for use in reverse osmosis. This type of membrane, first fabricated from cellulose acetate, is capable of separating micro-molecular and ionic materials from water. Over the last 30 years, this technology using hydrophilic membranes has matured into a robust array of products and is used as a means of water purification for millions of gallons per day, providing both drinking water and highly purified reprocessed water for use in the electronics industry.
For use in processing organic compositions, the technology to date has been based on the use of composite membranes having a porous support layer to provide physical support for the active membrane film, the thin layer that provides means for separation of the species of interest. Composite membranes fabricated for separation application may be formed in any number of ways. Often the manner in which membranes are formed is governed by the physical form in which the membrane is applied, such as flat sheet, disc or spiral wound forms, all of which tend to be commercially prepared by a method in which an inert support layer is coated with a solution of polymer by various methods and dried to evaporate the solvent. Polymeric hollow fiber membranes can be spun from spinnerets, much like any synthetic fiber, to form a porous support. The support is then coated by dipping or, in a more advanced technology, plasma coated with a thin nonporous active membrane that serves as the separation membrane.
Several patents disclose membranes constructed of glassy polymers. These membranes are usually used for gas separation. For example, U.S. Pat. Nos. 4,880,441 and 4,935,141 disclose membranes made of glassy polymers or of more than one polymer. U.S. Pat. No. 5,147,550 to Wijmans discloses membranes for use in a pervaporation process. However, he does not make an asymmetric, hydrophobic membrane composed of an elastomeric polymer, though he does lists several such polymers in a list of polymers for making all membranes. However, no asymmetric hydrophobic membranes prepared from elastomeric polymers are disclosed in the prior art.